empty sheetname will cause damaged file

[eaglexiang]

Description

Steps to reproduce the issue:

1. Run this demo
2. You get one damaged file
3. MS Excel cannot open this file unless you fix sheetname of workbook.xml inner test.xlsx file.

package main

import "github.com/xuri/excelize/v2"

func main() {
	f := excelize.NewFile()
	f.NewSheet("")
	err := f.SaveAs("test.xlsx")
	if err != nil {
		panic(err)
	}
}

Describe the results you received:

I got a damaged excel file.

Describe the results you expected:

Err returned by Method NewSheet or other Export-class Methods.

Output of go version:

go version go1.18.3 windows/amd64

Excelize version or commit ID:

v2.6.1

Environment details (OS, Microsoft Excel™ version, physical, etc.):

Microsoft® Excel® 适用于 Microsoft 365MSO (版本 2208 Build 16.0.15601.20148) 64 位

[eaglexiang]

Adding err to NewSheet is simple but broken, Adding err to Export-class Methods like SaveAs and WriteTo may cause too many changes. So I gave up contributing this time. :)

[xuri]

Thanks for your issue, I have fixed it. Please upgrade to the master branch code, and this patch will be released in the next version.